The present invention relates to a structure of light emitting diode (LED) and a method of making the same, and more particularly, to a structure of lateral current blocking LED and a method of making the same.
In recent years, a great deal of attention has been directed to the light-emitting device utilizing gallium nitride-based semiconductors such as GaN, AlGaN, InGaN, and AlInGaN. Usually, most of the light-emitting devices of the aforementioned type are grown on an electrically insulating sapphire substrate, which is different from other light-emitting devices utilizing conductive substrates. Since the sapphire substrate is an insulator, the electrodes cannot be directly formed on the substrate, and has to directly contact the P-type semiconductor layer and the N-type semiconductor layer individually so as to complete the manufacturing of the light-emitting device formed on the sapphire substrate.
Please refer to FIG. 1A showing the cross section viewed along the a-axe2x80x2 line in FIG. 1B and FIG. 1B showing the top view of the conventional nitride LED. The structure shown in FIG. 1A and FIG. 1B can be formed via the following steps. Firstly, a buffer layer 20 is epitaxially grown on a substrate 10, wherein the material of the substrate 10 is such as sapphire; and the material of the buffer layer 20 is such as AlN or GaN. Then, a semiconductor layer 30 of a first polarity (made of material such as (AlxGa1-x)yIn1-yN(0xe2x89xa6xxe2x89xa61;0xe2x89xa6yxe2x89xa61)), a cladding layer 40 of the first polarity (made of material such as (AlxGa1-x)yIn1-yN(0xe2x89xa6xxe2x89xa61;0xe2x89xa6yxe2x89xa61)), an active layer 50 that has double heterostructure or quantum well and comprises (AlxGa1-x)yIn1-yN(0xe2x89xa6xxe2x89xa61;0xe2x89xa6yxe2x89xa61), a cladding layer 60 of a second polarity (made of material such as (AlxGa1-x)yIn1-yN(0xe2x89xa6xxe2x89xa61;0xe2x89xa6yxe2x89xa61)), and a highly doped contact layer 70 of the second polarity (made of material such as (AlxGa1-x)yIn1-yN(0xe2x89xa6xxe2x89xa61;0xe2x89xa6yxe2x89xa61)) are sequentially epitaxially grown on the buffer layer 20.
Afterwards, the aforementioned epitaxial layers are etched or polished via dry etching, wet etching, or mechanical cutting and polishing, thereby exposing a portion of the semiconductor layer 30 of the first polarity. Then, a metal electrode pad 90 of the first polarity is deposited on the exposed portion of the semiconductor layer 30 of the first polarity via thermal evaporation, e-beam evaporation, or sputtering, etc.; and a transparent electrode 100a of the second polarity and a metal electrode pad 100b of the second polarity are sequentially deposited on the contact layer 70 of the second polarity.
Although the transparent electrode 100a of the second polarity of the aforementioned structure can enhance the effect of current spreading, but in fact, most of the current still flows along the line between the transparent electrode 100a of the second polarity and the metal electrode pad 90 of the first polarity, causing no current flowing through most of the active layer 50, so that the light emitting efficiency of LED is not high (the light emitting region being mostly concentrated between the transparent electrode 100a of the second polarity and the metal electrode pad 90 of the first polarity), and the life of LED is reduced (due to the current overly concentrated causing temperature too high in local region). Although the thickness of the transparent electrode 100a of the second polarity can be increased so as to improve the effect of current spreading, yet the transparency of the transparent electrode 100a of the second polarity is reduced consequently.
Moreover, if the photon generated by the active layer 50 is emitted to the surface of LED at a large angle, the loss of total reflection will occur easily, so that only the photon emitted at a large angle from the neighborhood of the lateral of LED can be emitted outward more easily from LED.
Therefore, there are many relevant patents about the aforementioned conventional techniques. For example, Toshiba addressed a method of re-growth to confine the current (U.S. Pat. Nos. 5,732,098/6,229,893), wherein an insulating layer is deposited in the semiconductor element so as to achieve the effect of confining the current vertically. However, the aforementioned steps are complicated, so that the cost is increased. LumiLeds utilized the etching of p metal electrode to increase the current distribution and the light emitting efficiency, thereby achieving the high light emitting effect (U.S. Pat. Nos. 6,291,839/6,287,947/6,258,618). However, the depth of etching is not enough, so that the current transmission path cannot be assured definitely. Boston University of U.S. addressed that the photonic crystal can be applied in LED (U.S. Pat. No. 5,955,749), but the disadvantage therein is that the depth of etching is too deep and the production thereof is difficult.
Just as described above, there are disadvantages about the conventional nitride LED. Therefore, an objective of the present invention is to provide a lateral current blocking LED and a method of making the same, wherein the etching of the trench used to block the lateral current can be performed simultaneously with the process of exposing the semiconductor layer of the first polarity (in order to make the metal electrode pad of the first polarity), so that the production cost is not increased.
Another objective of the present invention is to provide a lateral current blocking LED and a method of making the same, wherein the trench is located between the two metal electrodes for increasing the possibility of the current passing through the active layer (the light emitting region), and the brightness of the LED.
Still another objective of the present invention is to provide a lateral current blocking LED and a method of making the same, and the trench thereof can be used to provide the chance of the photons emitted from the lateral of the trench, wherein the photons are generated from the active region at the central region of the element, especially for enabling some photons that originally would be totally reflected to be emitted out from the lateral of the trench via the trench, thereby increasing the output efficiency of the photons generated from the active layer.
According to the aforementioned objectives of the present invention, the present invention provides a lateral current blocking LED comprising: a substrate; a semiconductor layer of a first polarity, wherein the semiconductor layer of the first polarity is located on the substrate; a semiconductor epitaxial structure, wherein the semiconductor epitaxial structure is located on one portion of the semiconductor layer of the first polarity, and comprises an active layer, the semiconductor epitaxial structure comprising at least one trench, wherein the depth of the at least one trench reaches to at least the active layer; a metal electrode pad of the first polarity, wherein the metal electrode pad of the first polarity is located on the other portion of the semiconductor layer of the first polarity; and a metal electrode pad of a second polarity, wherein the metal electrode pad of the second polarity is located on the semiconductor epitaxial structure, and the metal electrode pad of the first polarity and the metal electrode pad of the second polarity are located at two opposite sides of the at least one trench.
According to the aforementioned objectives of the present invention, the present invention further provides a method of making a lateral current blocking LED, the method comprising the following steps: firstly, providing a substrate; then, forming a semiconductor layer of a first polarity on the substrate; then, forming a semiconductor epitaxial structure on the semiconductor layer of the first polarity; then, removing a first portion of the semiconductor epitaxial structure, thereby exposing one portion of the semiconductor layer of the first polarity; then, removing a second portion of the semiconductor epitaxial structure, thereby forming at least one trench in the semiconductor epitaxial structure, wherein the semiconductor epitaxial structure comprises an active layer, and the depth of the at least one trench reaches to at least the active layer; afterwards, forming respectively a metal electrode pad of the first polarity and a metal electrode pad of the second polarity on the exposed portion of the semiconductor layer of the first polarity and a third portion of the semiconductor epitaxial structure, wherein the metal electrode pad of the first polarity and the metal electrode pad of the second polarity are located at two opposite sides of the at least one trench.